The invention relates to a device and a method for temperature compensation by a determination of the cut-angle of crystals used in oscillators. The invention relates also to device and a method for determining the identity (or difference) of the cut-angle of crystals used in oscillators.
In particular, the invention addresses to problems how an effective temperature compensation can be achieved in a clock oscillator without the need of performing an estimation procedure which requires a temperature cycling during a production process. The temperature compensation technique of the present invention is particularly useful for all applications where two clock oscillators each having a crystal are used, e.g. in multistandard applicationsxe2x80x94one example being a GSM-AMPS dual mode operation). Since the temperature compensation using two xe2x80x9cmatched crystalsxe2x80x9d requires two crystals, it is most suitable for applications that need to have two reference frequencies for operation. For example, two reference frequencies in a dual mode subscriber station operating in the GSM and GPS systems, require two different frequencies of 13 MHz and a selected reference frequency (the company SiRF e.g. has selected 24.5525 MHz). However, the invention is not restricted to use in reference oscillators and it may be used for any type of oscillator for any particular application purpose.
Generally, in such crystal oscillators the oscillation frequency temperature characteristic is mainly determined by the resonant frequency temperature characteristic of the respective crystals and the invention is particularly directed to compensate this resonant frequency temperature characteristic of crystal oscillators. However, any other circuit element in the two crystal oscillators can be matched according to the invention in order to cancel out temperature effects of such other circuit components. Therefore, although the invention will hereinafter be explained with reference to the crystal matching of crystals used in oscillators, the inventive concept is generally applicable to matching of other circuit components which determine the resonance frequency temperature characteristic of the oscillators.
Crystals (crystal oscillators) are fundamental to radio communication equipment and to many other electronic circuits which require a very stable resonant frequency. The very high quality factor (Q=10000 . . . 5000000) and a small temperature coefficient make them most attractive as the frequency determining elements in frequency oscillator circuits. A particular useful crystal is a AT-cut crystal which covers a fundamental frequency range from 0.5 MHz to 30 MHz. Most of the considerations below are made for the AT-cut crystals, however, it should be noted that the invention is not restricted to this special type of crystal. Other crystal types may be used as well.
When using crystal oscillators in radio communication equipment, the long term stability due to temperature dependent frequency drift is important. Firstly, it is desired that the oscillator keeps a predetermined frequency stable over a long period of time independent from temperature changes. Secondly, if there is a frequency drift due to ambient temperature variations, it is desired that a well defined control is carried out to tune the frequency back to the desired frequency. For doing so, it is necessary to have an exact knowledge of the AT-cut crystal temperature characteristic. Of course, the resonant frequency temperature characteristic of the crystal oscillator is not only determined by the resonant frequency temperature characteristic of the crystal itself but also from additional active circuitry which may be used in the oscillator in addition to the crystal. Hereinafter, it is, however, assumed that the resonant frequency temperature characteristic of the crystal oscillator is mainly dependent on the resonant frequency temperature characteristic of the crystal itself.
As already indicated above, in the prior art several solutions are available in order to obtain a well defined temperature characteristic of a crystal. The temperature characteristic must be known also to perform the temperature compensation in case the temperature changes during operation. FIG. 1 shows an overview of the relation between the available temperature range, initial frequency deviation, frequency drift over temperature and cost for a standard 13 MHz crystal. As shown in FIG. 1, a small initial frequency deviation and a small frequency drift over temperature may only be obtained at very high cost. For reasonable costs (see the first line for 100% cost) a quite large initial frequency deviation of xc2x130 ppm and a large drift over temperature of xc2x150 ppm must be expected. Whilst the selection of a crystal with a low frequency drift over temperature may be a straight forward solution to build crystal oscillators with low frequency drift over temperature, this approach is certainly a very highly cost intensive one.
As already explained above, instead of just selecting a crystal at very high cost, an alternative approach is to select a moderately priced crystal and to provide the oscillator with a frequency control input (e.g. voltage controlled) and use this frequency control input as a temperature dependent control signal to apply a control voltage tuning the frequency of the oscillator back to the desired frequency dependent on the temperature changes. For example, a voltage controlled variable capacitor can be used in the oscillator circuit in order to pull the resonance frequency (frequencies) of the crystal and thus of the oscillator back to the desired frequency. Using voltage controlled crystal oscillators, different temperature compensation principles can be distinguished.
A first approach uses a pre-detuned control signal voltage, where pre-detuning is a function of the temperature and where the pre-detuning is selected to counteract the frequency drift of the crystal. Such temperature dependent control signals can for example be generated by a NTC/PTC resistor network.
For a digitally compensated crystal, the temperature response of the crystal (if known) is digitized and stored in a look-up table. The control voltage used for tuning the resonant frequency of the oscillator is adjusted according to the current ambient temperature which is sensed by a temperature sensor and the values stored in the look-up table. Both types of crystal oscillators are widely used and are known as VCTCXO and DCTCXO.
Two further solutions for achieving a temperature compensations of crystals (or crystal oscillators) are to place the crystals (or the crystal oscillators) into a temperature controlled oven. Since the temperature is usually kept constant well above the maximum ambient temperature, the crystal itself is operating at a constant temperature and hence independent of the ambient temperature. This achieves a lowest temperature dependent frequency drift.
Another approach is to use a feedback loop to compensate temperature effects. In this case, the crystal oscillator is synchronized to a common frequency standard (master clock) like the DCF77 or is synchronized to a base station like in the GSM system. That is, crystal oscillators used in such feedback loops (e.g. in a GSM network) may use the network reference for temperature compensation purposes assuming that the network reference is available and stable whenever the temperature compensation is required.
Whilst usually the temperature dependent resonance frequency drift is unwanted and a temperature compensation is applied to retune the resonant frequency, an advantageous use of the temperature dependent frequency drift is for temperature sensing purposes where a strong temperature dependence of the resonant frequency is desirable. However, obviously for performing a precise tuning of the resonant frequency or for an accurate temperature measurement, a precise knowledge about the frequency drift versus temperature is necessary. That is, the more precise the temperature characteristic of the crystal is known (measured), the better the temperature compensation or the temperature sensing will be. However, as will be explained below, it is difficult or at least cost intensive to have such a precise knowledge of the temperature characteristic of the crystal when it is purchased from the manufacturer.
A crystal can be regarded as a thickness-shear vibrator in which all oscillators knots are located inside the resonator. For a large resonator and small electrodes all the oscillator energy is focused at the center of the disc, i.e. a low damping fixture of the crystal at its circumference. The resonance frequency is then determined mainly by the effective elasticity and the resonant frequency of the fundamental mode in an AT-cut crystal can be expressed as f=1.660/d where d is the thickness of the crystal disc. Generally, the temperature characteristic of such an AT-cut crystal is a third order curve with an inflection point which lies typically between 25xc2x0 C. and 35xc2x0 C. depending on the actual cut-angle and the mechanical construction of the crystal disc. The third order temperature characteristic df/f generally expressed as
df/f=A1(Txe2x88x92Tref)+A2(Txe2x88x92Tref){circumflex over ( )}2+A3(Txe2x88x92Tref){circumflex over ( )}3xe2x80x83xe2x80x83(1)
can be reduced if one refers to the inflection point temperature Tinv instead of the initial temperature Tref leading to
df/f=a1(Txe2x88x92Tinv)+a3(Txe2x88x92Tinv){circumflex over ( )}3xe2x80x83xe2x80x83(2)
where:
df/f [ppm]=(f(T)xe2x88x92f(Txx))/f,
Txx=Tref,
Txx=Tinv,
frequency deviation
a3=1.05E-5
a1=xe2x88x920.84*dphi
dphi=phi_zzxe2x88x92phi_0
phi_zz=cut-angle
phi_0=zero angle
T=temperature
Tinv=inflection point temperature
Tref=reference temperature
As can be seen from equation (2), the steepness of the inversion point depends on the cut-angle (phi_zz) of the crystal. dphi is the difference between the cut-angle and a so-called zero angle phi_0 where the temperature characteristic has a horizontal tangent at the inflection point. By the choice of the appropriate cut-angle, two inversion points appear in the temperature characteristic, a maximum below Tinv and a minimum above Tinv. The temperature gradient is zero at each of these inversion points. Furthermore, the crystal diameter and the crystal surface curvature affect the temperature characteristic and the inflection point temperature. FIG. 2 and FIG. 3 are a look-up table and a graph, respectively, showing values of df/f according to equation (2) for different values of the angle dphi (and therefore also implicitly for the cut-angle).
For example, if the cut-angle dphi (actually dphi is the difference value, however, for simplicity hereinafter dphi is also called the cut-angle and it is assumed that the zero angle phi_0 is known) is known to be xe2x88x924 and the temperature is 5xc2x0 C. a frequency deviation of xe2x88x927.5600 ppm occurs andxe2x80x94by applying a control signal to a voltage controlled oscillatorxe2x80x94this frequency deviation could be appropriately compensated to retune the oscillator to the desired center frequency. For example, if an oscillator having a tuning sensitivity of 1 ppm/volt is used, then a control voltage of xe2x88x927.5600 volt would have to be applied.
When buying a crystal from the manufacturer, many parameters of the crystal such as the diameter and the thickness can be predetermined. However, the only thing that is not precisely known, but is needed for a precise temperature compensation is the cut-angle since it can not be measured on the crystal itself. Therefore, the actual temperature characteristic of the crystal is not known. More specifically, if the cut-angle was known immediately the temperature characteristic would also be known and the temperature compensation could be carried out. Since it is not known other techniques must be used in order to determine the actual temperature frequency characteristic of the crystal as will be explained below.
For the determination of the frequency drift of an AT-cut crystal over a temperature range it is usually sufficient to measure the frequency deviation at three different temperatures. For higher accuracy, five or even more different temperature points are needed. The overall temperature response can be derived by an interpolation technique. Additional tuning steps can be used in order to optimize the overall temperature characteristic of the crystal and the predistortion network, e.g. in NTC/PTC network compensated oscillators or digitally compensated oscillators. The tuning process is needed because the desired temperature characteristic of the crystals may not be preserved over large numbers of devices and furthermore NTC/PTC network is affected by tolerances.
Frequency measurements carried out at different temperatures are, however, time-consuming and require suitable equipment which makes temperature compensated crystal oscillators considerably more expensive than ordinary oscillators. Furthermore, the temperature characteristic of the crystal must be matched to the predistortion network by a tuning or trimming process in order to achieve an improved overall temperature characteristic. This can be very tedious and can require several steps in the production process and therefore adds undesirable costs to the product.
As explained above, the determination of the resonant frequency temperature characteristic df/f of a crystal is essential in order to perform an accurate temperature compensation or an accurate temperature sensing procedure. However, the exact determination of this temperature characteristic and a corresponding tuning network may increase the cost or may even be inaccurate since it is not possible to measure the frequency temperature characteristic with high accuracy. On the other hand, if the cut-angle was known then the look-up table in FIG. 2 or a graph in FIG. 3 or in fact the numerically stored equation (2) could be used for an accurate determination of the temperature characteristic and thus for a highly precise temperature compensation and temperature sensing. However, the cut-angle of the selected crystal can not be measured as such.
A first object of the invention is to provide a device and a method for determination the cut-angle of crystals in order to allow a highly precise temperature compensation in oscillators.
A second object of the invention is to provide a device and a method which allow to determine the difference or identity of the cut-angles of crystals selected from different batches.
The first object is solved by a device for temperature compensation via a determination of the cut angle of crystals used in oscillators, comprising a first crystal oscillator including a first crystal, which is cut under an angle and which has a first resonant frequency temperature characteristic, adapted to output a first oscillation frequency with a first oscillation frequency temperature characteristic determined by said first resonant frequency temperature characteristic, and including a first tuning means for tuning said first oscillation frequency in response to a first control signal, said first tuning means tuning said first oscillation frequency to a predetermined first center frequency when said first control signal has a first default value, at least one second crystal oscillator including a second crystal which is cut under the same angle as said first crystal and which thus has the same resonant frequency temperature characteristic as said first crystal, adapted to output a second oscillation frequency with a second oscillation frequency temperature characteristic and including a second tuning means for tuning said second oscillation frequency in response to a second and third control signal said second tuning means tuning said second oscillation frequency to a predetermined second center frequency when said second control signal has a second default value, and said second oscillation frequency temperature characteristic being identical to said first oscillation frequency temperature characteristic when said third control signal is disabled and being detuned thereto when it is enabled, a second crystal oscillator setting means for enabling and setting said third control signal to a control value dependent of a temperature, a processing means including a first/second control signal setting means for setting said first control signal and said second control signal to their default values, at least one frequency ratio determining means for determining a frequency ratio parameter representing the frequency ratio of said second oscillation frequency to said first oscillation frequency when said first and said second control signals are set to their default values and said third signal is enabled and set to said control value corresponding to said temperature value, a memory means for storing a known relationship of the frequency ratio parameter dependent on the temperature and the cut angle and an access means for accessing said memory means with said temperature and said determined frequency ratio parameter and for reading out the cut angle corresponding thereto.
Furthermore, the first object is solved by a method for temperature compensation via a determination of the cut angle of crystals used in a first and at least one second crystal oscillator having a first tuning means and a second tuning means for tuning the first and second oscillation frequencies in accordance with a first and a second and a third control signal, wherein the first oscillation frequency temperature characteristic and the second oscillation frequency temperature characteristic are identical when said third control signal is disabled and are different if said first control signal is enabled, comprising the following steps: storing in a memory means a known relationship of a frequency ratio parameter dependent on the temperature and the cut angle, disabling said third control signal and tuning said first and said second oscillation frequencies to their center frequencies by setting the first and the second control signals to default values, enabling said third control signal on and setting said third control signal to a control value dependent of a temperature, measuring the first oscillation frequency and the second oscillation frequency and determining a frequency ratio parameter representing the ratio of the second to the first oscillation frequency, accessing the memory means with said determined frequency ratio parameter and said temperature and reading out the cut angle corresponding thereto.
The second object is solved by a device for determining the identity of the cut angle of crystals used in oscillators, comprising: a first crystal oscillator including a first crystal, which is cut under an angle and which has a first resonant frequency temperature characteristic, adapted to output a first oscillation frequency with a first oscillation frequency temperature characteristic determined by said first resonant frequency temperature characteristic, and including a first tuning means for tuning said first oscillation frequency in response to a first control signal, said first tuning means tuning said first oscillation frequency to a predetermined first center frequency when said first control signal has a first default value; a second crystal oscillator including a second crystal which is cut under an angle and which has a resonant frequency temperature characteristic; adapted to output a second oscillation frequency with a second oscillation frequency temperature characteristic; and including a second tuning means for tuning said second oscillation frequency in response to a second control signal, said second tuning means tuning said second oscillation frequency to a predetermined second center frequency when said second control signal has a second default value; said second oscillation frequency temperature characteristic being different to said first oscillation frequency temperature characteristic due to a difference in cut-angle of the first and second crystals; a processing means including a first/second control signal setting means for setting said first control signal and said second control signal to their default values; a frequency ratio determining means for determining a frequency ratio parameter representing the frequency ratio of said second oscillation frequency to said first oscillation frequency at a measurement temperature different to the inflection point temperature of the crystals; memory means for storing a known relationship of the frequency ratio parameter dependent on the cut-angle of the first crystal and the cut-angle of the second crystal at said measurement temperature; an access means for accessing said memory means with said determined frequency ratio parameter and for reading out the cut angles of the first and second crystal corresponding thereto; and a calibration means for determining the identity of the cut-angles on the basis of the read out first and second crystal cut-angles.
Furthermore, the second object is solved by a method for determining the identity of the cut angle of crystals used in a first and a second crystal oscillator having a first tuning means and a second tuning means for tuning the first and second oscillation frequencies in accordance with a first and a second control signal, comprising the following steps: setting said first control signal and said second control signal to default values for tuning the first and second oscillation frequencies to their center frequencies; storing a known relationship of the frequency ratio parameter dependent on the cut-angle of the first crystal and the cut-angle of the second crystal at said measurement temperature in a memory means; measuring the first oscillation frequency and the second oscillation frequency and determining a frequency ratio parameter representing the ratio of the second to the first oscillation frequency at a measurement temperature different to the inflection point temperature of the crystals; accessing said memory means with said determined frequency ratio parameter and reading out the cut angles of the first and second crystal corresponding thereto; and determining the identity of the cut-angles on the basis of the read out first and second crystal cut-angles.
The object of the invention is also solved by claims 26, 27, 28, 29, 30, 31, 32, 33. According to a first aspect of the invention a xe2x80x9cmatched crystal conceptxe2x80x9d is used, i.e. the of use two crystals with identical (but unknown) cut-angles which have the same temperature characteristic such that the resonant frequency versus temperature characteristic will be identical. A first and second oscillator are used each having a crystal with the identical cut-angle but with different resonant frequencies. Since both crystals have identical cut-angles, the temperature characteristic of the first and second oscillator will be the same, however, without the knowledge of the cut-angle itself the temperature characteristic cannot be known. A third control signal is applied to the second oscillator to cause an intentional detuning of the temperature characteristic of the second oscillator with respect to the frequency characteristic of the first oscillator. A frequency ratio parameter of the resonant frequency of the first oscillator to the resonant frequency of the second oscillator in the detuned state is used to read out the cut-angle from a memory at the temperature which corresponds to the setting of the third control signal. The determined cut-angle can then be used to determine the temperature compensation needed.
Therefore, the information about the cut-angle is essentially derived by actively detuning the temperature characteristic of the second oscillator with respect to the first oscillator and a parameter is determined which essentially corresponds to the difference of the two temperature characteristics.
According to the second aspect of the invention, if two oscillators are used having two different crystals with different cut-angles (i.e. not from the same batch), the frequency ratio parameter is determined at a measurement temperature and based on the determined frequency ratio parameter the difference of the cut-angle and/or the identity (symmetry), respectively, of the two crystals can be determined.
Further advantageous embodiments and improvements of the invention can be taken from the dependent claims. However, it should be noted that the invention also comprises embodiments consisting of features which have been separately described in the description and the claims. Therefore, what will be presented below is only the preferred mode of the invention as presently conceived by the inventors. Further advantageous embodiments can be derived by the skilled person on the basis of the teachings and the disclosure herein.